1. Field
Various exemplary embodiments of the present invention relate to a semiconductor design technology and, more particularly, to a semiconductor memory device that performs refresh operations and a refresh control system.
2. Description of the Related Art
Semiconductor memory devices such as double data rate synchronous dynamic random access memory (DDR SDRAM) include memory banks for storing data and each memory bank includes memory cells. Each memory cell has a cell capacitor and a cell transistor and the semiconductor memory device stores data by charging or discharging the cell capacitor.
It would be ideal if the charge in the capacitor remained constant. However, the charge stored in the cell capacitor varies due to voltage differences between the semiconductor memory device and peripheral circuits. That is, the electrical charge may leak from a charged capacitor or be unintentionally charged when it should remain discharged. This unwanted charging and discharging may result in the loss of data. To combat this concern, semiconductor memory devices perform refresh operations to keep their charges at intended levels and prevent the loss of stored data.
The degree of integration of memory devices is constantly increasing and this means the size of memory cells and memory banks have to decrease as well. Also, the distance between memory cells and their corresponding word lines is also decreasing. This has resulted in an increase in electrical phenomena that affect the memory cell charges. For instance, there may be electrical coupling effects between adjacent word lines.
In a semiconductor memory device, word lines must become active (increase or decrease in voltage) to read or write from/to memory cells. As the distance between word lines decreases, this coupling effect is intensified and memory cells connected to adjacent word lines may have their charge (voltage level) influenced due to the coupling effect. Since data is stored by the charge, this may result the loss of data.
To prevent loss of data, memory cells are refreshed to keep the memory cell charges at or near their intended levels. This must be before the charge in the memory cells has been lost. However, it should not been done more than necessary as refresh operations deteriorate the operating efficiency of semiconductor memory devices. Therefore, much effort is spent on perfecting refresh operations to ensure data is accurately maintained while keeping the memory device operating as efficiently as possible.